Although the invention described herein can be used in many ways, a particular method of use is in what is popularly called sous vide cooking. The French term “sous vide” is widely used in English to refer to a method of slowly cooking food in vacuum sealed plastic pouches, but the term is also used more generally to refer to a slow method of cooking using precisely controlled low temperature water. For example, the sous vide literature includes references to cooking eggs in the shell without the need for a sealed pouch. The term “sous vide” literally means “under vacuum,” and it is sometimes used in the more literal sense to refer to the process of creating the food-filled vacuum sealed pouches. Accordingly a sous vide process can refer to the method of sealing food in the bags or to the cooking process. Except where noted otherwise herein, the term sous vide will be used to mean the cooking method and is not intended to be limited to the case where sealed pouches are used.
Sous vide typically differs from conventional cooking methods in two ways: 1) the raw food is usually vacuum sealed in appropriate plastic pouches, and 2) the food is cooked in the sealed pouch using precisely controlled low temperature in water. Although the relevant process for the invention typically uses raw food, it is also possible to partially or fully cooked food. When precooked food is used the process is normally called reheating rather than cooking.
Temperatures used in sous vide cooking are lower than those normally used for cooking foods by frying or in boiling water. Most sous vide cooking is performed between 130 to 180 degrees Fahrenheit (° F.) with a typical temperature being around 140° F. (60° C.), which is well below the boiling point of water. Even though the temperature is low, it is, nevertheless, important that the temperature be well controlled. While some foods will cook in a few minutes, sous vide cooking time can extend to tens of hours. The purpose of the lower temperature and longer cooking duration is to maintain the integrity of the food ingredients, improve the taste over conventionally prepared foods and induce biochemical reactions which improve the quality of the food, for example, to improve the tenderness of lesser cuts of meat. The long cooking times allow the food item to reach the desired equilibrium temperature with the heated water without any significant internal temperature variations, which is not the case for high temperature cooking methods.
Examples of recommended slow cooking temperatures and time reveals that the temperature needs to be precisely controlled, but the time is given as a large interval. For example, for a “tough cut” of beef medium rare the specification might be a final internal temperature of 149° F. and the cooking time given as 16 to 24 hours. To cook the same “tough cut” to rare might be specified as 136° F. and 24 to 48 hours. For a better cut of beef steak the final internal temperature might be 129° F. for rare and 136° F. for medium rare with a cooking time of 1.5 to 3 hours. While the texture of some foods such as fish may become mush-like after extended cooking times, the upper time limit for the slow cooking method is not limited to even the expansive values given above for many food as long as the internal temperature remains capped. Accordingly one prior art temperature controller for sous vide applications is said to control the temperature to with 0.1° F.
As the sous vide name suggests, vacuum sealing in a plastic bag is generally used to remove air from contact with the food. The exclusion of air from the cooking bag is not required for cooking but has some advantages. It allows cooked food to be stored longer. Another advantage of using a vacuum sealed cooking bag is the exclusion of oxygen. Reducing oxidation during the extended cooking time can be particularly important for cooking meat, for example, because the fat may become rancid with prolonged exposure to oxygen in the air at elevated temperature. Reducing the level of oxygen also dramatically decreases the opportunity for aerobic bacteria growth in the food. In addition, the vacuum condition reduces the tendency of the sealed bags to float, and improves the efficiency of contact between the cooking fluid and the food.
An example of a particular sous vide cooking pouch is described in U.S. Pat. No. 6,056,985 to Fluckiger et al. (May 2, 2000). The material for the bag is flexible and transparent which allows direct observation of the food while it is cooking. Fluckiger uses heat-shrinking material that can be heated to at least the pasteurization temperature of the food therein. He suggests a temperature tolerance up to about 110° C. Fluckiger includes a special self-sealing orifice in the bag that allows insertion of a temperature sensor to measure the temperature inside the food during heating and cooling phases. One end of the sensor with an attached wire remains outside the bag to allow the data to be acquired and used by the control system. The pass-through orifice in the bag re-seals when the temperature sensor is removed.
The required degree of accuracy and constancy of cooking temperature varies with the food and cooking time. However, sous vide style cooking can benefit from a cooking device that can maintain a precise temperature over long periods of time.
There are several types of sous vide style cooking devices that have been described. One way to divide the devices is whether they provide a means for actively circulating or stirring the liquid. The use of circulation pumps, for example, can provide better control over the temperature. But many units omit circulation means and rely on convection presumably to save on the complexity and/or cost.
One type of device that includes circulation capability consists of an integrated unit that attaches to the rim of a general purpose cooking pot. The user interface panel, process control electronics and pump are positioned above the level of the liquid. Tubes which provide intake and output ports for a circulation pump extend down into the liquid.
U.S. patent application 20080066624 by Taylor, et al. (published Mar. 20, 2008) describes a sous vide cooker with an integrated immersion circulator. The appliance comprises a vessel for holding a liquid bath, one or more heater units, an immersion circulator, one or more temperature sensors, and a user input device. The sensors monitor the temperature of the liquid bath and provide feedback to the heating units and the immersion circulator. This feedback allows the heating units and immersion circulator to maintain the temperature at the desired setting. The heating chamber is open on the top, allowing the introduction of the liquid comprising the liquid bath and vacuum packed food items for cooking. The heating chamber is equipped with a drain for removing the liquid.
A cooker that relies on convection is described in U.S. patent application 20110185915 by Eades; et al. (published Aug. 4, 2011). The sous-vide cooker includes a cooking chamber and a heating system. A plate with holes in it is said to create passive circulation. The heating system comprises a heating unit, one or more temperature sensors, a proportional-integral-derivative (PID) controller, and a control panel.
U.S. Pat. No. 5,542,344 to Koether et. al. discloses a sous vide rethermalizer appliance including two tanks in a fixed arrangement. The first tank is used for heating sous vide pouches to a temperature of approximately 140° F. The second tank is used for “finishing” where the sous vide pouches are cooked for a short time period at approximately 180° F. to complete cooking of the food in the pouches prior to serving. Each tank includes a heater and relies on convection. Water is automatically added to the tanks when appropriate by the control system. A timing and memory unit retrieves set point temperatures corresponding to the designated vat functions.
These prior art sous vide cooking devices either include a detachable circulation means that is immersed into the general purpose cooking vessel by the user or include a dedicated cooking vessel with a permanently attached circulation means.
The above described prior art devices have a number of short-comings when compared to the present invention.